Preparation and pyrolysis of satu



Patented Nov. 6, 1951 PREPARATION AND PYROLYSIS OF SATU- RATED ACETALS OF CROTONALDEHYDE Robert H. 'Saunders, Oxford, Pa., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Application December 13, 1947,

Serial No. 791,666

This invention relates to l-alkoxybutadienes and to the method for their preparation from crotonaldehyde.

The preparation of l-alkoxybutadienes from crotonaldehyde has heretofore been accomplished by reacting crotonaldehyde with alcoholic hydrogen chloride to form the acetal of betachlorobutyraldehyde and subsequently heating this acetal with a strong alkali. This process is not entirely satisfactory because ofpoor yield of products and because of the added cost of hydrogen chloride and sodium hydroxide.

In accordance with the present invention, l-alkoxybutadienes are produced moreeconomically and in greatly improved yields by condensing crotonaldehyde with an alcohol to form, an acetal of crotonaldehyde and subsequently pyrolyzing the acetal.

The condensation of crotonaldehyde with an alcohol to form the acetal of crotonaldehyde is carried out by heating at a temperature between about 80 C. and about 200 C. The water may be removed within this temperature range as an azeotrope with one of the reagents or as an azeotrope with an added water-immiscible liquid boiling within the temperature range of the condensation. The water is preferably removed progressively as the condensation proceeds.

The alcohol which is condensed with the crotonaldehyde should be chosen according to the particular alkoxybutadiene desired since the alkoxy group will have the same number of carbon atoms as the alcohol used. Broadly, any of the saturated aliphatic alcohols having not more than 18 carbon atoms such as methanol, ethanol, stearyl alcohol, etc., may be used in this invention. However, the water-immiscible primary and secondary saturated aliphatic alcohols having from four to eight carbon atoms such as the primary and secondary butyl, amyl, hexyl, heptyl, and octyl alcohols are preferred.

The pyrolysis of crotonaldehyde acetal to pro- I duce a l-alkoxybutadiene may take place at any temperature within the range of about 150 C. and 300 C. Within this range one alkoxide group is 7 Claims. (Cl. 260-614) 2 tillation flask with a spiral packed distillation column. An azeotrope containing crotonaldehyde and water distilled at a head temperature of 84 C. The temperature in the pot was about 110 C. The crotonaldehyde was separated from the water in the distillate and returned to the flask. After about four hours distillation, during which time water was separated from the distillate, the butanol was distilled off and the dibutyl acetal of crotonaldehyde was recovered by distillation in vacuo. The dibutyl acetal of crotonaldehyde boiled at 68 C. (1.0 mm. pressure) and had a refractive index C. D line) .4283. The yield was substantially quantitative.

When the above preparation was repeated with crotonaldehyde freed of acid by washing with sodium carbonate and an inert atmosphere was used, the dibutyl acetal was obtained in just as good yield but more time was required for com- 20 pletion of the condensation.

One hundred thirty-seven parts dibutyl acetal of crotonaldehyde was pyrolyzed at 210-220 C. (pot temperature) in a flask having a fractionating column. The heating was so regulated that 26 vapors distilling over did not have a vapor temsplit off as alcohol which may be separated. and

the l-alkoxybutadiene may be recovered.

The following examples illustrate the processes of this invention in greater detail but the invention is not restricted to these examples. The parts are by weight.

Example I A mixture of 148 parts butanol and 140 parts (20 C. D line) 1.4578. It showed an ultraviolet absorption band at 237 m having an extinction coefiicient of 190. On hydrogenation the butoxybutadiene yielded dibutyl ether.

Example II A mixture of 725 parts -hexanol, 1'75 parts crotonaldehyde, and 435 parts benzene was refluxed in a 10 plate bubble cap fractionating column. The pot temperature was about C. The benzene water azeotrope was slowly distilled off and the excess reagents were then removed in vacuo. The dihexyl acetal of crotonaldehyde distilled at 94-96 C. (0.5 mm. pressure). Its refractive index (25 C. D line) was 1.4366. The yield was substantially quantitative.

The dihexyl acetal of crotonaldehyde was-added dropwise to a flask containing alumina pellets heated at 260-290 C. The vapors which distilled from the flask were condensed and redistilled to commercial crotonaldehyde was refluxedin a 66 separate hexyl alcohol from the product. The

3 1-hexoxybutadiene fraction boiled at 84-93 C. (16 mm. pressure) and had a refractive index C. D line) 1.4590. It showed an ultraviolet 7 absorption band. at 237 m, with an extinction coefiicient. of 128.

Exam le III a A mixture of 600 parts isopropyl alcohol, 14 parts crotonaldehyde, and 130 parts benzene w'as' refluxed for '72 hours while the water.-was moved as an azeotrope with benzene fhe w was separated from the azeotrope and the organic portion was returned to thejsYstEmflThe Hi 1 isopropyl acetal of crotonaldehyd'e was isolated. by distillation of the product as a f1action, v of which boiled at 63-65 Cflfl'l fri'imiir'ssffifgfl" 1, and had a refractive index (25 0. D line) raise.-

The diisopropyl acetal of crotonaldehyde when slowly distilled. at atmospheric pressure yields an iscprepyl; alcohol solution of l-i sopropoxyloutadierie. en 'tlie iuitigl idiet absorption item at 33'; mu whichischaracteristic df'thfe 1- a ii i r e esa v Whi le-t he temperature for s am formation; is not criticaland be anywhere within the range of aboutsm C. "and ahout"209 "0., the [preferred temperature is within the range of a 0. andabout C.

The water liberated in the reaction it form V theacetal is; progressiyely removed in order to shift the equilibrium. The water may he sepa- -rated by distillation as an azeotrope with the crotonaldehyde, "with-the "alcohol,,-or with an -added azeotropic agent-such as-benzene, toluene,

Gh,3heptane,;andthe like; [ar as amixed azeo- "l5 the''azeotrope and they =are -returned to the re--.-

action. *In the case where-a -highenboiling alcoihol is iused-as a reagent, it is desirableto-have an iadded lower bOllln'g azeotrope to aid; in the removel; of the water, especially during-the latiter stages of the reaction when -much--of =the crotonaldehyde has already underonereaction. H w W v v p I,

sired, reduced'pressure may be used during the Usually the amount of -a1cohol -taken for reaction-with the crotonaldehydewill bein excess 'cf' that' theoretically required. Moreover-, alarge exc'essrofalcohol over that requiredfor the 1';e-- "action'may be used. Eoweven ther-e-is-generally -no advantage in using more 'thanaboutd-q moles alcohol per mole crotenaldehyde. A e-tad {hole excess of alcohol is preferred when the alcohol removed along with the water is returned continuously to the system. ,7

- Catalysts which have hertofdfe bee iisedfin t condensation of 'crotonaldehydeiiiithalco-' hols haveiled tag-products other than mascara of crotonaldehyde. For example, when hifdfoa mild catalytic efiect for the formation of the acetals of crotonaldehyde but not for the addition of the alcohol to the double bond. Acids of this type, however, are not effective catalysts becausei they combine with the alcohol much more rapidly than the crotorfialdehyde in the process "of this invention; and any catalytic effect is fleeting. Therefore, carboxylic acids are not classified as catalysts within the scope of this invention nia Ide pT'eSErit amounts normally present i insne'r'eaaents;

I he a'cetals maybe distilled or otherwise purified, -if des'ired dr' they may be used directly ;-;without; further ;purification for the pyrolysis step;

For example, the excess alcohol and fctfdnaidehyde, if-any be present, may be dish tilled from the crotonaldehyde acetal and the temperature may be raised to the pyrolysis range for the second step in the process. Although the 20 sa etalmay be-tormedat a temperature within the wanear-a utawe o, to about;200 C. which is within the pyrolysis temperature range, the presence 'Qf alcohol required. for acetal formation ?-suppresses :the pyrolysis reaction thus allowing -onhz -sma ll amountsof alkoxybutadiene to=form.

The pyrolysis reaction "may be carried out at any teiripera ture within the range C. to -;300 C. It is usually carried'outhy heating the fcrotonaldeh-yde acetal Within this range and 30 separating byitheir higher volatility the products of the reaction as they for-m. 'ilhe separation Jnay suitabiy'be effected byany'of the usual frac- -tionatingcolu1nns 1 or fractionating condensers. *In suohaipr'ocessthe entirecharge of crotonaldehyde ac'etal :inay be'heated'at once or the acetal may he 'added gradually to the .LpyI'OIy-SiS vessel. The pyrolysis may likewiseeb'e efiected by :pyrm z-mg and then fractionally distilling to h auetsrrom s'tarting mat'erial. In pyrolysis is incomplete d-u'e to 'the set up-under 'con'd-itionsof pyrolysis oiiin he alcoh'ol formed. The re- 'el ial ma y b'e subjected to gain. 'In-any-of these yar-ipyrolysis pyroly i t feme e-the pyrolysis products. The "pre erred pyrolysis temperature is-within the range "drabe ur 1 7c Giana-n70 c.

. 1 fCa'talys'ts are not required "for "thefpyrolysis. 5 new vei',catalysts.such*ais silica-gel,"metal acid Y'metaphosphate's, 'aci'd clays, magns -iuhi.aluminiifnsilioats, and"the like maybe "urea. *wnentathystsarefuseu, a decrease or the teifiperatfifeandiiont tiniewith tarefurcontrol thereof during thefiiyfo'lysisfis frequireuirer pest ,rsuits. Masai/er, -.';the alkr'S-iZyiiiitaHiene rproducdiin the. pyrolysis il'iniis't be -1""er'r1oved from pp a e rrest r -Within? ctehi's'tio-rfiifiifiiiz rpelymerizaticn. Because of ease of contrel', noncatalytiepyi olysisis peifferred. V

U 'I hepyrolysis reaction-' involved in this invenl T'-'br-ium reaction whichfisshiited etionpy-rernoval of: one ofi-theiproducts.

ct is mrz 7 Usually the alcohol liberated in t-he pyrclysis eing lower boiling than the l-alkoxybutadiene, is progressively remo'i edsbyrsdis tillation.

On the reagents, crotonic acid, for example, may have "is zirwntitrrtnrareageasstmaemaamaa,

for example. If the l-alkdxybutadiene is to be recovered, however, the alcohol is preferably distilled 011 as the pyrolysis reaction proceeds.

The l-alkoxybutadienes produced by the process of this invention are valuable intermediates for modified rubbery polymers. They form especially valuable copolymers by copolymerization with styrene, methyl vinyl ketone, methyl methacrylate, and the like. They are also highly useful intermediates for the production of plasticizers by-condensation with maleic esters.

In accordance with this invention, it is seen that there is thus provided a method of producing crotonaldehyde acetals and a method for producing l-alkoxybutadienes therefrom, each of which methods constitutes a novel, simple and economical process and there is also provided by the combination of the two methods a novel, simple, and economical process for producing the l-alkoxybutadienes from crotonaldehyde without creating useless by-products of the prior art processes.

What I claim and desire to protect by Letters Patent is:

1. In the process of producing a l-alkoxybutadiene from crotonaldehyde by condensing crotonaldehyde with a saturated aliphatic monohydric alcohol and pyrolyzing the crotonaldehyde acetal so formed, the step of preparing the crotonaldehyde acetal by heating crotonaldehyde with a saturated aliphatic monohydric alcohol at a temperature within the range of about 80 C. and about 200 0., in the absence of a catalyst, while progressively distilling off the water evolved.

2. In the process of producinga l-alkoxybutadiene from crotonaldehyde by condensing crotonaldehyde with a saturated aliphatic monohydric alcohol and pyrolyzing the crotonaldehyde acetal so formed, the step of preparing the crotonaldehyde acetal by heating crotonaldehyde with a saturated aliphatic monohydric alcohol at a temperature within the range of about 100 C.

and about 130 C., in the absence of a catalyst, while progressively distilling ofi the water evolved.

3. In the process of producing a l-alkoxybutadiene from crotonaldehyde by condensing crotonaldehyde with a saturated aliphatic monohydric alcohol and pyrolyzing the crotonaldehyde acetal so formed, the step of preparing the crotonaldehyde acetal by heating crotonaldehyde with a saturated aliphatic monohydric alcohol at a temperature within the range of about 80 C. and about 200 C., in the absence of a catalyst, while progressively distilling off the water evolved as a constant boiling mixture with one of the components of the reaction mixture.

4. In the process of producing a l-alkoxybutadiene from crotonaldehyde by condensing crotonaldehyde with a saturated aliphatic monohydric alcohol and pyrolyzing the crotonaldehyde acetal so formed, the step of preparing the crotonaldehyde acetal by heating crotonaldehyde with a saturated aliphatic monohydric alcohol at a temperature within the range of about 100 C. and about 130 C., in the absence of a catalyst, while progressively distilling off the water evolved as a constant boiling mixture with one of the components of the reaction mixture.

5. In the process of producing a l-alkoxybutadiene from crotonaldehyde by condensing crotonaldehyde with a saturated aliphatic monohydric alcohol and pyrolyzing the crotonaldehyde acetal so formed, the step of preparing the OTC-,- tonaldehyde acetal by heating crotonaldehyde with a saturated aliphatic monohydric alcohol and an inert ,volatile water-immiscible liquid at a temperature within the range of about C. and about 200 C., in the absence of a catalyst, while progressively distilling off the water evolved as an azeotrope with said water-immiscible liquid.

6. In the process of producing a l-alkoxybutadiene from crotonaldehyde by condensing crotonaldehyde with a saturated aliphatic monohydric alcohol and pyrolyzing the crotonaldehyde acetal so formed, the step of preparing the crotonaldehyde acetal by heating crotonaldehyde with a saturated aliphatic monohydric alcohol and an inert volatile water-immiscible liquidat a temperature within the range of about C. and about C., in the absence of a catalyst, while progressively distilling off the water evolved as an azeotrope with said water-immiscible liquid.

7. The process of preparing a l-alkoxybutadiene which comprises the steps of heating crotonaldehyde with a saturated aliphatic monohydric alcohol in the absence of a catalyst at a temperature within the range of about 80 C. and about 200 C. and, without isolating the crotonaldehyde acetal so formed, continuing to heat the reaction mixture to a temperature within the range of about C. and about 300 C., the water and alcohol being progressively distilled off as they are evolved in the process.

ROBERT H. SAUNDERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,931,858 Bauer Oct. 24, 1933 2,124,686 Carothers July 26, 1938 2,189,529 Carothers Feb. 6, 1940 OTHER REFERENCES Dupire Chem. Abstracts, vol. 37, page 3737. Abstracts of article in Compts. Rendus, vol. 214 (1942) pages 359-360.

Faucett, "Paint, Oil 8: Chemical Review, June 8, 1939, pages 9-11.

Buttline, Paint, Oil & Chemical Review, May 6, 1943, page 15.

Beilstein, Handbuch der 0 r g a n i s c h e n Chemie, vol. 1, supplement, page 108. 

1. IN THE PROCESS OF PRODUCING A 1-ALKOXYBUTADIENE FROM CROTONALDEHYDE BY CONDENSING CROTONALDEHYDE WITH A SATURATED ALIPHATIC MONOHYDRIC ALCOHOL AND PYROLYZING THE CROTONALDEHYDE ACETAL SO FORMED, THE STEP OF PREPARING THE CROTONALDEHYDE ACETAL BY HEATING CROTONALDEHYDE WITH A SATURATED ALIPHATIC MONOHYDRIC ALCOHOL AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 80* C. AND ABOUT 200* C., IN THE ABSENCE OF A CATALYST, WHILE PROGRESSIVELY DISTILLING OFF THE WATER EVOLVED. 